Process for c8 aromatic feed fractionation

ABSTRACT

D R A W I N G MULTIPLE SEPARATION IN A SINGLE FRACTIONATION ZONE OF C8 AROMATICS INTO COMPONENT ETHYLBENZENE, O-XYLENE AND M-AND/OR P-XYLENE UTILIZING THE SAME HEAT FLOW. THE FEEDS TO THE FRACTIONATION ZONE COMPRISE A FIRST FEED RELATIVELY RICH IN ETHYLBENZENE AT A FIRST INTERMEDIATE ZONE LEVEL AND A SECOND FEED RELATIVELY RICH IN O-XYLENE BELOW THE FIRST FEED LEVEL. BOTH FEEDS MAY BE OBTAINED FROM AVAILABLE PROCESS STREAMS OR BE FORMED FROM A SINGLE FEED BY PRELIMINARILY FRACTIONATING THE FEED IN ADVANCE OF THE COMPLEX FRACTIONATION. IN EITHER CASE THE FEED MAY BE DEPLETED OF MXYLENE PRIOR TO FRACTIONATION BY CHEMICALLY COMPLEXING THE M-XYLENE INTO A SEPARABLE FORM E.G. WITH HF-BF3. THE OVERHEAD STREAM, A PREDOMINANTLY O-XYLENE BOTTOMS STREAM AND A SIDESTREAM PREDOMINANTLY OF M-XYLENE AND/OR P-XYLENE. THE SIDESTREAM MAY BE PROCESSED TO SEPARATE P-XYLENE AS A PRODUCT OF THE PROCESS, AND/OR TO CONVERT UNWANTED ISOMERS TO DESIRED ISOMERS, WITH SUCH CONVERTER EFFLUENT BEING RETURNED DIRECTLY TO THE FRACTIONATION ZONE, OR TO THE FEED, FOR FRACTIONATION IN COMMON WITH THE FRESH FEEDS.

PROCESS FOR Ca AROMATIC FEED FRACTIONATION Filed March 23, -19'70 S. B.JACKSON EVAL June 8, y1971 4 Sheets-Sheet 1 I l J A/Evs/ ENDS?- wf i@ N4m f. j lllllllllli'l M N ,zw/, D U MW wm m y@ m mw E 7\/ lllll 2J aw/MM im 2. wl? (4 L a 0 ,/L e p y D 0 ,n f. F

s u W rse N 3M Mw I.. W BH E e fr@ E G i zw s m Lm E Ep i w lw p N 4Naww @E j r 1 1mm. X/ 9/ m-aw TQ n am e 7 5 a J ,Zwmf FPJ f /.C L Hipaa..w @5 p q June 8, 1971 s. B. .JAcKsoN EVAL 3,584,068

PROCESS FOR C8 AROMATIC FEED FRACTIONATION Filed March 23. 1970 4Sheets-Sheet z T 70m/Eux June 8, 19"/1 s, B JACKSON EIAL 3,584,068

PROCESS FOR C8 AROMATIC FEED FRACTIONATION June 8, 1971 s. B. JACKSONETAL 3,584,068

PRocEss FOR of', ARoMATxc FEED FRACTIONATION 4 Sheets-Sheet 4 FiledMarch 23, 1970 IN VEA/Tow. STE vE/v B J2? c/sa/v .R 0G52 Maen#@from/E915.

3,584,068 Patented June 8, 1971 U.S. Cl. 260-668 77 Claims ABSTRACT OFTHE DISCLOSURE Multiple separation in a single fractionation zone of C8aromatics into component ethylbenzene, o-xylene and mand/or p-xyleneutilizing the same heat flow. The feeds to the fractionation zonecomprise a rst feed relatively rich in ethylbenzene at a firstintermediate zone level and a second feed relatively rich in o-xylenebelow the irst feed level. Both feeds may be obtained from availableprocess streams or be formed from a single feed by preliminarilyfractionating the feed in advance of the complex fractionation. Ineither case the feed may be depleted of mxylene prior to fractionationby chemically complexing the m-xylene into a separable form e.g. withHF-BF3. The fractionation zone provides a predominantly ethylbenzeneoverhead stream, a predominantly o-xylene bottoms stream and asidestream predominantly of m-xylene and/or p-xylene. The sidestream maybe processed to separate p-xylene as a product of the process, and/or toconvert unwanted isomers to desired isomers, with such converter eluentbeing returned directly to the fractionation zone, or to the feed, 'forfractionation in common with the fresh feeds.

REFERENCE TO RELATED APPLICATION This application is a continuation inpart of our copending application Ser. No. 653,177, led July 13, 1967,and now abandoned.

BACKGROUND OF THE INVENTION (l) Field of the invention This inventionhas to do with the separation from a predominantly CB aromatichydrocarbon feed, such as is obtainable from catalytic reforming ofnaphtha, of various commercially desirable component materials incontrollable ratios, with low capital cost and with low utilitiesconsumption. Chief among desired materials so obtainable areethylbenzene, o-xylene and p-xylene. The fourth C8 aromatic isomer,m-xylene, is commercially less important and its output accordingly maybe minimized. Other feed components, such as non-aromatic hydrocarbons,toluene, C9 aromatics, etc., also must be separated insofar as isrequired to meet quality specifications on the various C3 aromaticproducts. The various components of C8 feeds can 'be heat separated,that is, advantage may be taken of their different boiling points tofractionate the feed, with lower boiling materials e.g. ethylbenzene(B.P. 277 F.) and light ends passing overhead out of a fractionatingzone and higher boiling materials e.g. o-xylene (B.P. 291 F.) and heavyends withdrawn as bottoms from the zone. The m-xylene and pxylenefractions may be withdrawn as overhead or bottoms depending on operationof .the zone. Generally, successive separations are carried out toisolate the different desired products. p-Xylene for example, may beseparated from m-xylene in a p-xylene crystallizing plant by freezingthe p xylene (F.P. 55 F. and passing the m-xylene (F.P. 54 F.) out ofthe plant. Because m-xylene has limited commercial usage, it is oftenisomerized, or converted, to p-xylene and/or oxylene, either precedingor following fractionation operations. This invention is particularlyconcerned with a complex :fractionation process having low capital costsand low operating expense by virture of the separation of theethylbenzene and o-xylene components from the m-xylene and p-xylenefractions ywith the same heat ow. Various preliminary and subsequentoperations -to the complex fractionation may be combined therewith togive extremely ilexible Ca processing abilities for maximum advantage inuse of available feeds to satisfy product requirements.

(2) Prior art Many different schemes have been proposed for thefractionation and conversion of predominantly C5 aromatic hydrocarbonfeeds. In general, it has been the practice to rst separate o-xylene andheavy ends together from the remainder of the feed, then in a separateoperation, the o-xylene and heavy ends are separated one from another.The remainder of the feed, in another separate operation, isfractionated to separate together ethylbenzene and light ends. Thebalance of the feed, comprising predominantly m-xylene and p-xylene maybe passed to a pxylene crystallization plant and on to a conversionplant to obtain first crystallized p-xylene and, secondly, additionalp-xylene formed from m-xylene. o-Xylene and heavy ends in the conversionplant eilluent typically are passed with other xylenes to still anotherfractionator where they are separated from p-xylene as bottoms, thesecond o-xylene and heavy ends separation in this operation. Thep-xylene rich remainder can be returned directly to the crystallizationplant .for processing with additional feed from the ethylbenzenefractionator. In another commercial process the m-xylene component ischemically separated from the feed, and optionally converted toadditional p-xylene and o-xylene, in advance of fractionationoperations, but the fractionation of remaining components is effected inmultiple steps with successive heatings and extensive handling ofseveral streams.

SUMMARY OF THE INVENTION One of the major objectives of the presentinvention is to achieve a variety of C8 products with fewer separateheating operations, thus to eliminate substantial capital expense andoperating costs. This objective is met by separating one from anotherwith a common heat flow in a fractionation zone, the major desiredfractions, ethylbenzene, o-xylene and m-/p-xylene. Specifically, thisobjective, and others to become apparent as the description proceeds arerealized by the present process for fractionating a predominantly C8aromatic hydrocarbon mixture containing ethylbenzene and xylenes intocomponent fractions which includes feeding a first C8 aromatics streamrelatively rich in ethylbenzene into a multilevel fractionation zone ata first feed tray located intermediate the top and bottom of the zone,feeding a second C8 aromatics feed stream relatively rich in o-xyleneinto the zone at a second feed tray located intermediate the top andbottom of the zone and below the first feed tray, and heating both feedstreams with a common heat fiow in said zone, and separating apredominantly ethylbenzene overhead stream, a predominantly o-xylenebottom stream and a xylene isomer sidestream at a level of the zonebetween the first and second feed trays.

The first feed stream, relatively rich in ethylbenzene, typicallycomprises at least 8 mol percent ethylbenzene; the second feed stream,relatively rich in o-xylene typically comprises at least 8 mol percento-xylene. The complex fractionation zone may be operated at temperaturesbetween 250 and 500 F., and pressures between 10 and 100 p.s.i.a.

The process in certain embodiments includes processing the xylene isomersidestream to isolate p-xylene therefrom, andprocesing the resultingp-xylene depleted sidestream in a conversion zone to convert m-xyleneisomer present therein to other xylene isomers, and returning theconversion zone effluent to the fractionation zone as the second feedstream relatively rich in o-xylene. The conversion zone may be operatedto convert m-xylene to o-xylene or p-xylene or both, with the commonheat fiow in the fractionation zone being utilized to enable separationof the o-xylene and p-xylene produced in the conversion zone from oneanother. The process may further include returning separated o-xylene tothe conversion zone for conversion into p-xylene, or conversion ofp-xylene to oxylene, as dictated by product requirements. Typically theamount of p-xylene isolated from the sidestream is at least equal to theamount of p-xylene present in the predominantly C8 aromatics mixture.

In certain embodiments, the present process further includes heating thepredominantly C8 aromatic mixture in a preliminary fractionation zone,and separating the feed into an overhead portion relatively rich inethylbenzene and a bottoms portion relatively rich in o-xylene, feedingthe overhead portion into a multilevel fractionation zone at a firstfeed tray located intermediate the top and bottom of the zone, feedingthe bottoms portion into the multilevel zone at a second feed traylocated intermediate the top and bottom of the zone and below the firstfeed tray, and heating both feed streams with a common heat flow in thelmultilevel fractionation zone, and separating a predominantlyethylbenzene overhead stream, a predominantly oxylene bottom stream anda xylene isomer sidestream at a level of the multilevel zone between thefirst and second feed trays. The preliminary fractionation typically iscarried out at temperatures in the range of 250 and 600 F. and atpressures between 10 and 150 p.s.i.a.

The first stream, overhead, from the preliminary fractionation is heatedincrementally in the multilevel fractionation zone to separateethylbenzene and light ends from the p-xylene and m-xylene in the firststream.

Heat utilized in the prefractionation is advantageously transferred fromthe prefractionation zone to the multilevel fractionation zone as may beeffected by cycling the preliminary fractionation zone overhead throughthe fractionation zone above the sidestream draw point or by reboilinghydrocarbons in the multilevel fractionation zone between the feedstreams against condensation of the preliminary fractionation zoneoverhead vapors outside the multilevel fractionation zone. Thehydrocarbons for yreboiling may be drawn from the multilevelfractionation zone at or near the level of the sidestream, wherebypreliminary fractionation zone heat is utilized in both stripping andrectification operations in the multilevel fractionating zone.

The foregoing embodiment of the present process may further includereturning the efiiuent from the conversion zone processing of p-xylenedepleted sidestream, fwherein m-xylene has been converted into o-xyleneand/or pxylene, to the multilevel fractionation zone below thesidestream level.

m-Xylene may be depleted from the initial feed to the process in advanceof fractionation in other embodiments of the process. Thus the presentinvention contemplates preliminarily depleting the C8 aromatichydrocarbon mixture of m-xylene, feeding a m-xylene depleted mixturerelatively rich in ethylbenzene into the multilevel fractionation zoneas a first feed stream at a first feed tray located intermediate the topand bottom of the zone, feeding a second C8 aromatics feed streamrelatively rich in o-xylene into the zone at a second feed tray locatedintermediate the top and bottom of the zone and below the first feedtray and heating both feed streams with a common heat flow in themultilevel fractionation zone, and seperating a predominantlyethylbenzene overhead stream, a predominantly o-xylene bottom stream anda xylene isomer side-stream at a level of the zone between the first andsecond feed trays. The m-xylene may be depleted from the feed bychemically modifying the m-xylene to a form separable from the feed, asby complexing the mxylene with a. complexing agent such as HF BF3.

The m-xylene initially depleted from the feed mixture may be heated toconvert the separated m-xylene to o-xylene and/or p-Xylene, theseconversion products then being fed to the multilevel fractionation zonefor separation in common with other C8 aromatics in that zone. In otherrespects, the initial m-xylene depletion embodiment of the invention issimilar to the embodiment described above and may include sidestreamprocessing to isolate p-xylene and return of the p-xylene depletedstream to the multilevel fractionation zone or at least a portionthereof may be combined with fresh feed ahead of the m-xylene depletionzone, isolating at least the amount of p-xylene present in the firstfeed stream from the sidestream, converting separated o-xylene orisolated p-xylene to p-xylene or o-xylene respectively to be fed to themultilevel fractionation zone e.g. at a point above the sidestream leveland also preliminarily fractionating the m-Xylene depleted feed mixtureand utilizing the preliminary fractionation heat as described above inthe multilevel fractionation zone.

In each of the foregoing embodiments the multilevel fractionation zonetypically is operated at temperatures between 250 and 500 F. and atpressures between 10 and p.s.i.a., the preliminary fractionation, ifany, at temperatures between 250 and 600 F. and pressures between 10 and150 p.s.i.a. The relatively ethylbenzene rich first feed streamtypically may contain from 12 to 25 mol percent ethylbenzene whetherderived from an available process stream or from the prefractionationoverhead product of a predominantly C8 aromatic feed mixture stream. Therelatively o-xylene rich second feed stream may contain from 15 to 30mol percent o-xylene and generally will contain at least 8 mol percento-xylene whether derived from available process streams, fromprefractionation bottoms, and/ or from converter efiiuent.

Also, in each of the foregoing embodiments, product qualityspecifications may require that the various products of the process bepurified further to remove unwanted contaminants, which purications areeffected by conventional methods not comprising an essential part ofthis invention. Typically, the ethylbenzene is purified by distillationand/ or solvent extraction, the o-Xylene by distillation, and thep-xylene by re-crystallization and/or selective adsorption, generallyincorporated within the p-xylene separation unit.

BRIEF DESCRIPTION OF THE DRAWINGS The invention will be furtherdescribed as to illustrative embodiments in conjunction with theattached drawings in which:

FIG. 1 is a schematic ow sheet of one embodiment of the present process;

FIG. 2 is a schematic flow sheet of an embodiment of the process inwhich the xylene isomer sidestream is processed to effect p-xyleneisomer separation and conversion and return of converter-effluent and/orother isomers to the fractionation zone and depicting ethylbenzene ando-xylene purification operations;

FIG. 3 is a schematic iiow sheet of an embodiment of the present processillustrating particularly the feed mixture prefractionation andintermediate reboiling of multilevel fractionation hydrocarbons againstcondensation of prefractionator overhead vapor;

FIG. 4 is a schematic flow sheet of an embodiment of the present processsimilar to FIG. 3, and particularly depicting another means of heattransfer from prefractionator to multilevel fractionator;

FIG. 5 is a schematic flow sheet of an embodiment of the present processgenerally similar to FIG. 4 and particularly depicting intermediatereboiling of multilevel fractionator hydrocarbons;

FIG. 6 is a schematic ow sheet of an embodiment of the present process,generally similar to FIG. 4, without special provision for heat transferfrom prefractionator to multilevel fractionator;

FIG. 7 is a schematic flow sheet of an embodiment of the present processparticularly depicting initial depletion of m-xylene from the feedmixture;

FIG. 8 is a schematic fiow sheet of an embodiment of the present processgenerally similar to FIG. 7 with the addition of preliminaryfractionation of the m-xylene depleted feed mixture.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 11n the ensuing descriptionlike numerals will refer to like parts in each ow scheme.

With reference first to FIG. l, a feed relatively rich in ethylbenzene,such as a fresh reformate fraction from catalytically reformed naphtha,is introduced into the system through line 1 as a first feed stream(Feed #1) to a fractionation zone defined by the complex fractionatorcolumn 2. The fresh feed may be suitably at a temperature of about 360F. and may flow directly from a toluene recovery fractionator (notshown). This first feed stream composition may typically be:ethylbenzene 16 mol percent; p-xylene 17 mol percent; m-xylene 42 molpercent; o-xylene 20 mol percent; and other hydrocarbons 5 mol percent.A feed relatively rich in o-xylene such as may be obtained as effluentfrom a xylene isomerization unit is introduced into the system throughline 3 as a second feed stream (Feed #2) to the complex fractionationcolumn 2, suitably at a temperature of about 400 F. The second feedstream composition may typically be ethylbenzene 8 mol percent; p-xylene21 mol percent; m-xylene 47 mol percent; o-xylene 21 mol percent; andother hydrocarbons 3 mol percent.

The complex fractionation zone in FIG. 1 is shown as a multilevel,plural tray column 2 which may alternatively be two or moreinterconnected sections. (See FIG. 2.) The first feed stream line 1enters the fractionator column 2 at a first feed tray 4 locatedintermediate the top and bottom of the column. The second feed streamline 3 enters the fractionator column 2 at a second feed tray 5 alsolocated intermediate the top and bottom of the column but below thefirst feed tray 4.

A multiple or complex fractionation is effected 1n column 2. In theupper portion of the column 2, ethylbenzene and light ends are separatedfrom m-xylene and p-xylene, while in the lower portion of the column,oxylene and heavy ends are separated from m-xylene and p-xylene.Importantly, these two different separations are effected with a singleor common heat flow in the column. 'Ihat is, heat input to the column 2by passage of column bottoms through reboiler loop 6 having heatexchanger 7 to raise the column bottoms temperature to about 250 F. to500 F., or higher, serves to enablerst separation of the o-xylene fromm-xylene and p-xylene, and, then, separation of ethylbenzene fromm-xylene and p-xylene higher in the column, as the heat travels upwardthrough the column. The overhead of the column 2', comprisingpredominantly ethylbenzene with light ends enters reux loop 8 havingcondenser 9 and accumulator 110. A portion of the condensed overhead inreflux loop 8 is returned to the column as reflux, the remainder ispassed along line 1v1 to ethylbenzene purification zone 12 where lightends are separated along line 13 and ethylbenzene product is taken outalong line 14.

A portion of the column 2 bottoms being continually passed throughreboiler loop 6` to maintain desired column temperatures, whichcomprises mainly o-xylene with heavy ends, is taken off along line 15out of the fractionation zone to o-xylene purification zone 16 whereinheaxy ends are separated through line 17 and the oxylene product ispassed out in line 1'8. With the separation of predominantlyethylbenzene and light ends overhead and predominantly o-xylene andheavy ends below, the remaining substantial components of the two feeds,i.e. m-xylene and p-xylene are separated to further processing alongline 19 leading from column 2 at a side draw tray level 20 between thefirst and second feed trays 4 and 5.

The column 2, as mentioned, may comprise two or more interconnectedsections and be coupled to ethylbenzene and o-xylene purification zone12, and i16. The column may further be connected to xylene isomerseparators and/ or converters. With reference to FIG. 2, such a fiowscheme is shown for processing a feed relatively rich in ethylbenzene,such as Feed #1 in the just described embodiment, introduced in thefractionation zone as the first feed stream. The second feed is obtainedfrom a xylenes converter and is relatively rich in o-xylene.

With reference to FIG. 2, the first feed stream enters the systemthrough line 1 at the complex fractionation zone defined by amultisection, plural tray column 2 having three interconnected sections,a lower section 2a, an intermediate section 2b and an upper section 2c.The first feed line 1 enters the intermediate section 2b at a feed-tray4. Within section 2b an interior temperature is provided which drivesthe light ends and ethylbenzene components of the feed upward to becarried from the intermediate section along line 21 to the bottom ofupper section 2c. In upper section 2c, the overhead from intermediatesection 2b is further fractionated so that the predominantlyethylbenzene overhead stream, containing the light ends of the feed istaken off overhead and passed into reflux loop `8 through condenser 9land accumulator 10 and into line 11 for separation of the ethylbenzene.The bottoms from upper section 2c are returned along line 22 to theintermediate section 2b.

The bottoms in intermediate section 2b, following passage overhead oflight ends and ethylbenzene and comprising primarily m-xylene, p-xylene,o-xylene and heavy ends are passed along line 23 from the intermediatesection for further fractionation. A portion of these bottoms is takenas a sidestream. along line 24 for processing outside of thefractionation column 2 to isolate the p-xylene in a manner to bedescribed, and the remainder is passed along line 23 to the top of lowersection 2a.

Alternatively, the sidestream may be taken from any location in thefractionation column 2 below the entry point of first feed stream line 1into the column (feed tray 4), with accompanying variation in theo-xylene content of the sidestream within the compositional o-xylenelimits of the two feeds to the column, for increased fiexibility ofproduct mix.

In the lower section 2a the bottoms are passed continually throughreboiler loop 6 and reboiler 7 to maintain section temperature atdesired levels. A portion of the bottoms, including o-xylene and heavyends, is taken off along line 15 out of the fractionation column 2 forseparation of o-xylene. The remainder of the lower section 2a feed,comprising primarily m-xylene and p-xylene is passed overhead along line25 to the bottom of intermediate section 2b.

It will be apparent that the three sections 2a, b, c are arranged to actas a single column with ethylbenzene and light ends taken out asoverhead, o-xylene and heavy ends taken out as bottoms and m-xylene andp-xylene separated as a sidestream.

The predominantly ethylbenzene overhead stream in line 11 isconventionally processed in the ethylbenzene purification zone 12 tostrip light ends, as necessary, for recovery of the desired purityethylbenzene. Or the overhead in line 11 may be processed by extractionor extractive distillation methods, as required when close-boilingnon-aromatic contaminants are present. Thus, the overhead stream ispassed along line 11 to column 26 wherein light ends are distilledoverhead to line 27 and partially returned as reux to column 26y throughcooling loop 28, including condenser 29 and accumulator 30 and passed towaste or preferably to return to the naphtha reformer or toluenerecovery column (not shown) for recovery of toluene or other values. Thecolumn 26 bottoms, predominantly ethylbenzene, is cycled throughreboiler loop 31 having heat exchanger 32 and a portion is passed toline 14 for recovery as a product of the process.

The bottoms from fractionation column section 2a in line isconventionally processed in the o-xylene separation zone 16 to separatethe o-xylene from the heavy ends. Thus, the bottoms stream is passedalong line 15 to column 35 wherein o-xylene is distilled overhead forrecovery through line 18, as a product of the process, from retluxingloop 37 having condenser 38 and accumulator 39. The heavy ends from thecolumn 35 are taken off as bottoms through reboiler loop 40 havingreboiler 41 to line 17, to Waste or further processing.

The sidestream taken from the intermediate section 2b of thefractionation column 2 is passed along line 24 to the p-xylene isolationzone. The p-xylene is conventionally isolated from its mixture withm-xylene e.g. by cooling in p-xylene separation zone 42 to crystallizethe higher melting p-Xylene component, and separating the p-xylene alongline 43. Alternatively the p-xylene can be selectively adsorbed orotherwise separated in zone 42. The liquid remaining, high in m-xylene,is passed through line 44 to the converter 4S. There, by well knowntechniques, not forming a part of this invention, the nonp-xyleneisomers may be converted to p-Xylene. The pxylene rich effluent fromconverter 45, unlike prior processes is not recycled for immediaterecovery of p-Xylene, following removal of heavy ends. Rather, theconverter effluent is passed along line 3 as the second feed to thefractionation column 2 used to fractionate fresh feed. The line 3 entersthe fractionator 2 at feed tray 5 in the lower section 2a and below thesidestream take-out line 23 from intermediate section 2b. Thus, the sidedra'w line 23 is between the two feed lines, i.e. between line 1carrying the first feed stream to the fractionation column 2 and line 3carrying the second feed stream comprising xylenes converter 45 effluentto the same fractionation column. Light ends produced in the converter45 may be withdrawn in part through line 46. In the fractionation column2, the converter 4S efiluent is fractionated much in the way the firstfeed stream is fractionated, and importantly with the same heat flow.That is, the p-xylene and o-xylene components are commingled with freshquantities of these materials and are eventually passed out of thefractionation column 2 with corresponding components from the freshfeed.

Importantly, conversion-produced o-xylene is fed along line 3 to thefractionation column 2 and is separated in that zone with fresh feedoriginated o-xylene for withdrawal from the column 2 along with heavyends in line 15. Thus, only one separation of o-xylene needs to be madeand both of the high utilities demanding steps, i.e. ethylbenzeneseparation and o-xylene separation, each from m-xylene and p-xylene, areaccomplished with the same heat flow.

A particularly advantageous aspect of the FIG. 2

embodiment of the present process is the flexibility of product mix.Referring to FIG. 2 it can be seen that oxylene product separated incolumn 35 can be returned along line 47 (dotted) to the xylene converter4S, to be incorporated in the converter feed in line 44 or fedseparately to converter 45 if desired, along line 34. Appropriatecontrol of the converter 45 operating conditions, in a known manner,converts the o-xylene at least partially to p-xylene for an increase inp-xylene yield at the expense of o-Xylene. Similarly, the m-xylenecontent of the sidestream in line 24 by appropriate control of theconverter operating conditions, in a -known manner, may be converted too-Xylene at least partially, for an increase in o-xylene production.Thus, greatly more or greatly less o-xylene can be produced than isprovided in the feed stock.

Where a C8 aromatic mixture containing substantial quantities of allfour isomers is available, another embodiment of the present process,depicted in FIG. 4 may be employed. As shown in FIG. 4, a feed mixturehaving typically the composition given above in connection with FIG. l,is introduced into the system along line 48 to the middle ofprefractionation column 49 in which column bottoms are heated bycontinual circulation through reboiler loop 50 having reboiler `51 totemperatures between 250 and 600 F. at pressures in the range of 10 andp.s.i.a., effectively preliminarily fractionating oxylene and somem-xylene and p-xylene in the process feed in line 48. Theprefractionator column 49 overhead which is relatively rich inethylbenzene is passed to line 1 as the first feed to the column 2 atfeed tray 4. The prefractionator column 49 bottoms which is relativelyrich in o-xylene is passed along line 3 as the second feed stream to themultilevel column 2 to enter the column at feed tray 5 below the firstfeed tray 4. As in the FIG. 1 embodiment, the first and second feedstreams in lines 1 and 3 are subjected to fractionation in column 2,with the ethylbenzene and light ends being separated from m-Xylene andp-xylene in the upper portion of the column and o-xylene and heavy endsbeing separated from mxylene and p-xylene in the lower portion of thecolumn. As in FIG. 1 and in the other embodiments of the process, thetwo different separations are effected with a single or common heat flowin the column 2 with heat input by reboiler loop `6 serving to enablefirst separation of oxylene from m-xylene and p-xylene and thenseparation of ethylbenzene from m-Xylene and p-xylene higher up in thecolumn as the heat travels upward. The overhead and bottoms of thecolumn 2 are taken off as described above in connection with FIG. 1.

Heat may be transferred from prefractionator column 49 to the complexfractionation column 2 by cycling prefractionator overhead through thefractionator column. Thus, column 49 vapor overhead in line 1 enterscolumn 2 just above the first feed tray 4 and liquid reflux smaller inquantity than the vapor overhead feed exits from column 2 in line 55from the feed entry level to give a net transfer of overhead productfrom column 49 to column 2, comprising Feed No. 1 to column 2, andtherewith a transfer of heat to column 2 from column 49.

In FIG. 3, the prefractionation in column 49 of a C8 aromatics feedstream from line 48 into an ethylbenzene rich overhead stream in line 1and an o-Xylene rich bottoms stream in line 3, as first and second feedstreams respectively for the complex fractionator 2 is carried outgenerally as described above in connection with FIG. 4. In this FIG. 3embodiment, however, intermediate reboiling of fractionator column 2hydrocarbons is provided. Thus, hydrocarbons in the fractionation column2 are with drawn from the column at a point between the feed trays 4 and5 and heat exchanged with the preliminary fractionation column 49overhead vapors, outside of the multilevel fractionation column 2. Toaccomplish this form of heat transfer from preliminary fractionationcolumn 49 two intersecting loops 52 and 56 are provided.

Loop 52 is a reflux loop for column 49 and includes combinationcondenser/reboiler unit 53 and accumulator 54. Loop 56 is anintermediate reboiler loop for column 2 and includes a side draw line 57passing through the reboiler side of unit 53 back to column 2 above line57 and below upper feed tray 4. Desirably the side draw line 57 is at ornear the level of the m-xylene/p-xylene sidestream line 24 so that theheat from preliminary fractionation column 49 overhead is utilizable inboth stripping and rectification operations in the upper part offractionation column 2. In the FIG. 3 embodiment, the xylene isomersidestream comprising predominantly m-xylene and p- Xylene is taken offin line 24 and processed to isolate p-xylene therefrom. The sidestreamis passed along line 24 to a p-Xylene separation zone 42 which may be acrystallizer as described above in connection with FIG. 2, or other typeof separator operating e.g. by selective absorption, complexing, orother technique to isolate pxylene. p-Xylene product is passed out ofthe separation zone 42 along line 43. The remainder of the xylenesisomer sidestrearn after separation of p-xylene, is high in coutent ofm-xylene and is passed along line 44 to converter 4S for production ofadditional p-xylene .and/or o-xylene in various ratios depending onmarket requirements. Light ends may be removed at least in part from theconverter eluent along line 46. Separated o-Xylene can be passed alongline 61 to the converter 45 for isomerization into p-xylene as desired.

In the FIG. 3 embodiment, as in the FIG. 2 embodiment above described,the effluent from converter 45 is recycled to the fractionation column 2along line 58 to column 2 at a point below the `sidestream take out line24. The fractionation column 2 thus has a supplementary feed or Feed #3relatively rich in p-xylene and/ or o-xylene which is simultaneouslyfractionated with the #1 and `#-2 feeds into component fractions with acommon heat ow in the column 2 provided by reboiler loop |6 andsupplemented by intermediate reboiler loop 56 as described above.Optionally, a portion or all of the converter 45 eiuent may be returnedalong line 59 to the prefractionator column 49 for processing there withfresh feed prior to fractionation in the column 2 or the converter 4Seffluent may be passed along line 60 to be combined with Feed #2(prefractionator bottoms) in line 3 downstream of the prefractionatorcolumn.

In FIG. 5 the process feed in line 48 having typically the compositiongiven above in the FIG. 1 embodiment description is prefractionated incolumn 49 to separate the process feed into an o-xylene rich bottomsstream in line 3 and ethylbenzene rich overhead stream in line 1. Heatinput to the column 49 is by reboiler loop 50. The column 49 overheadvapor is condensed on the condenser side of combined condenser-reboilerunit 53 in reflux loop 52 against liquid intermediate level hydrocarbonsin intermediate reboiler loop 56 vaporizing the liquid hydrocarbons onthe reboiler side of the unit 53 for ret-urn t0 the multilevelfractionation column 2. The condensed overhead vapors rich inethylbenzene are passed in part to column 2 in line 1 as Feed #1 and inpart returned to column 49 as reflux through loop 52. Product separationfrom column 2 in this FIG. 5 embodiment is as in the FIG. 1 embodiment,above described.

In FIG. 6, an embodiment of the present process, particularly usefulWhere fuel costs are low, is depicted. In this embodiment specificprovision for double utilization of prefractionator heat input is notmade which may be acceptable with low fuel costs. In FIG. 6 the processfeed in line 48 is prefractonated in column 49 as in the FIG. 5embodiment, just described. Reflux loop 64 including condenser 65 andaccumulator 66 returns a portion of the condensed overhead to column 49and a portion is passed along line 1 to column 2 as the first feedstream. The bottoms in column 49 are circulated through reboiler loop 50and a portion thereof is passed along line 3 to the multilevelfractionation column 2 as the second feed l0 stream. Operation of themultilevel fractionation column 2 and product recovery therefrom is asdescribed in connection with FIG. l above.

The process in the further em-bodiments shown in FIGS. 7 and 8 includesa preliminary treatment of the process feed to separate m-xylene and/orisomerize m-xylene to o-Xylene and/or p-xylene for the fractionationoperations; in reference to FIGS. 7 and 8, a feed of mixed xylenes andethylbenzene appropriate for fractionation (and conversion) to producep-xylene as well as ethylbenzene and o-xylene, as above described, isintroduced into the system through line l67. The feed passes to am-xylene separator and isomerizer 68 in which m-xylene is complexed witha complexing agent. The m-xylene complex may be separated from thebalance of the feed, then decomposed and the m-xylene all or in partisomerized to oand/ or p-xylene.

Typically, the separator and isomerizer 68 is a known unit whichcontains HF`-BF3 with which the feed from line 67 is intimatelycontacted at about 32 to 45 F. which results in essentially all them-xylene in the feed forming complexes with HFBFB in an HF solution. Them-xylene complex is passed as an HF solution to the isomerizer portionof separator 68 where the complex is heated to a temperature at which itis unstable and decomposes, freeing the m-xylene for recovery as suchalong line 70, and restoring the HF-BFS complex to its originalcondition for reuse. Or the m-Xylene-HF-BF`3 complex may be passed assuch to the isomerizer portion of separator and isomerizer 68 and atleast a portion of the m-xylene component isomerized to oand/orp-xylenes using the HF-BF3 present as a catalyst. The isomerizationproducts are then combined with the m-xylene depleted feed in line 1 toincrease the o-xylene and p-xylene content thereof for processing in thefractionation column 2. Optionally, light ends may be separated at leastin part in separator 68 and passed out of the system in line 69.

fIn FIG. 7, the m-xylene depleted feed is passed to the multilevelfractionation column 2 as a first feed stream to be heated within thecolumn to simultaneously separate ethylbenzene from oand p-xylenes at anupper level of the Zone and p-xylene from o-xylene on a lower level ofthe column. The use of a single heat ow in accordance with the teachingof this invention to effect these two separations in a m-xylene depletedfeed is a departure from previous practice in processing m-Xylenedepleted CB feeds. a

It will be noted that in the FIG. 8 embodiment of the invention apreliminary fractionation of the m-xyleue depleted feed in line 48 isaccomplished as described in connection with FIG. 5 above. Thus, them-xylene depleted feed in line 48 is passed into column 49 and separatedinto an ethylbenzene rich portion overhead to line 1 and to thefractionation column 2 as the first feed stream and an o-xylene richportion as bottoms along line 3 to the fractionation column 2 as thesecond feed stream.

As in previous embodiments, within multilevel column 2, light ends andethylbenzene components of the column feed are driven upwards to becarried as vapors into the upper reaches of the column where theethylbenzene is essentially completely separated from p-Xylene byfurther fractionation so as to pass the ethylbenzene (and light ends)overhead to reflux loop 8 and to pass p-xylene downward. In FIG. 8, theheat flow utilized in the preliminary fractionation of the feed incolumn 49 derived from heat exchanger 51 is transferred to multilevelfractionation column 2 through the combining condenserreboiler unit 53in reux loop 52 and intermediate reboiler loop 56 described above, to becombined with the heat flow in multilevel column 2 from reboiler 7 sothat the two heat flows are available to facilitate the relativelydifficult separation of p-xylene and ethylbenzene, where heat demand isgreatest.

Pressures within the columns 49 and 2 in the FIGS. 7 and 8 embodimentsas in other embodiments herein are not narrowly critical and typicallyrange between 10 and 150 p.s.i.a., but are not limited to this rangewhere economic considerations suggest other pressure levels.

Product recovery in the present embodiments is accomplished as in FIG. 1with respect to the ethylbenzene and o-xylene products. Thus overheadfrom column 2 enters reux loop 8, is condensed in condenser 9 and ispartly returned to column 2 as reux and partly passed along line 11 tothe ethylbenzene purication zone 12 for separation of light ends alongline 13 and separation of the ethylbenzene in line 14. Similarly theo-xylene bottoms stream from column 2 is passed along line 15 too-xylene 1 2 EXAMPLES For processing a typical Ca-aromatics richfeedstock having a composition of approximately:

lb.-mols/hour Heavy ends (C9-H Apparatus may be arranged according toany of the ow sheets FIGS. l through 8, the choice of flow sheet beingdependent upon the desired products of the process, the

purification zone 16 where heavy ends are separated along 15 relativeamounts of such products, and desired extent of line 17. The o-xyleneproduct is taken out in line 18. flexibility to change the products inresponse to changing In FIG. 8 the p-xylene sidestream is taken in line19 market conditions. The chosen apparatus may include, in just abovetray 20 in column 2 and passed to p-Xylene addition to the complexfractionator, one or more of the separator 42. The p-xylene product ispassed along line following: a prefractionator; a p-xylene separationunit; 43 out of the system, and the o-xylene rich remainder of 20 aXylefleS SOmeTiZation Unit With o1" Without mXYene the sidestream 19 ispassed from separator 42 back io Separati0r1; an ethylbenzene puricationunit; and an 0- multilevel fractionation column 2 along line 58 assupxylene Puflealon unltplementary Feed #3 to the column. Optionally, apor- EXAMPLE 1 tion or all of the p-xylene separator eluent in line 58 rmay be returned along line 72 (dotted) to the isomerizer 2') A processfor fractionation and conversion of the above 68 for conversion ofexcess xylene isomers from one to fnentioned typical feedstock may beeondueed accord' another, or along line 72 and thence along line 73(dotted) mg t0 the FIG' 2 OW Sheet to obtaln three Pnmafy Prod' to theprefractionation column 49 to enhance overall ucts as follows: recoveryof ethylbenzene product, or along lines 72 and 30 h (f1) EthyllbenzeequYalent to lbout 95 of that 1.n 73 and thence along line 74 (dotted)to be combined in t e ee fstgcj M19 0 mlnlmlm plglty ogtmmf a man; line3 with prefractionator bottoms as part of Feed #2 tIgTX/len m0 perce 1gt en s an mo percen to column 2. I'f p-xylene product is desired at theexpense (2) O Xy'lene equivalent to about 85% of that in .Ehe ofo-xylene product, separated o-xylene product 1n lllle feedstock, at 95%minimum purity, containing a maxi- 18 may be returned alongline 75(dotted) to isomeri'zer 35 mum of 4.0 m01 percent other CSHM materialand 1.0 68 for conversion to additional p-xylene product. Simmolpercentheavy ends. ilarly separation of less than all of the p-Xylene in sep-(3) p Xylens at maximum economic yield and 99% arator 42 enables returnof p-xylene along line 72 to the minimum purity, consistent with thestated production of isomerizer 68 for conversion, if desired, toadditional ethylbenzene and o-xylene, and one further requirement OXylene product 40 that the by-product C9+ heavy ends contain about 30mol .In FIG 7, the O Xyleue rich emuent from the p xylene percentxylenes. For this case, the apparatus includes (l) a Separator 42following Separation of p xylene product multisection complexfractionating column lh avingabout along line 43 is returned tomultilevel fractionation col- 1252 theormcal Stags. 370 actual traysdwlded Mito a' umn 2 as ,Feed #2 along line 3' Optionally a portion 45ower section comprising a column of 70 trays, an intermediate sectioncomprising a column of 150 trays, and an but not au of the P'XyieneSeparator emuent m lm e 3 upper section also comprising a column of 150trays, havmay be remmef along 1m@ 72 (dott'd) to the lsomenzer ing areux ratio of 14.0 (fresh feed basis), operable at 68 for ConverslonOfhexss xylene lsomers from .one t0 15.7 p.s.i.a. (accumulatorpressure), and capable of a heat aHOthef, 0r along line 72 and thencealong llne 73 input of 136.0 MM B.t.u./hour; (2) an ethylbenzene puri-(dotted) to be Combined in line 1 With Feed #1 '[0 CO1- 50 cation columnhaving about 26 theoretical stages, 40 umn 2 to enhance overall recoveryof ethylbenzene prodactual trays, a reflux ratio of 1.5, operable at15.7 p.s.i.a., uct. As in FIG. 8, additional p-xylene product may be andCapable of a heat input of 2-5 MM B-11-/ hour; (3) obtained by returningseparated o-xylene product in line an o'xylene Purcation Column havingabout 35 theo' 18 along line 7S (dotted) to isomerizer 68 for conversion55 retlcal Stages 55, actual trays a reflux rat10-0f 32 oper' to PXylene or additional O Xylene p 1 0 duct may be ob able at 15.7p.s.i.a., and capable of a heat input of 19.4 tained by separation ofless than all of the p-xylene in MM B't'l-l/hour; (4) a p-XyleneSeparauon umt m Whlch p-xylene is separated by crystallization; and (5)a xylenes Separator Hond retu'fn of .a Pomon of the effluent misomerization unit in which ni-xylene and excess o-xylene line 3 alongline 72 to isomerizer 68 for conversion of the are Converted to p Xy1eneA material balance at various unrecovered p-xylene to o-xylene. 60points in the process is given in Table 1.

TABLE i Complex fi'actonator (Column 2) o-Xylene purilcation (Unit 16)aan (zarten esta wenn) masas ComponentI pound mols per hour:

C7 3.95 1.09 Etliylbenzene 91. 84 89.03 128. 77 03 03 p-Xyiene 9s. 09 3i437. 42 i. i3 1. ii o2 m.Xy1em 241. i2 i1 100s. s4 3. 0s 2. 9s i0 OXylemh 117. 0s 33s. 99 12o. 93 99. 52 21.41 @9+ 32.97 26. 42 51. 27 1.0i50.26

Total 579. 37 93. 40 1941. 53 17s. 44 104. 65 71. 79

TABLE l-Contiuued Ethylbenzene purification p-Xylene separationIsomerization (Unit 12) nit 42) (Unit 45) Distillate Bottoms p-XyleneFiltrate (to Post light ends light ends ethylbenzene product converter)removal-recycle (Line 27) (Line 14) (Line 43) (Line 44) (Line 3)Comionent, pound mols per hour: 3 76 19 01 1 08 4 77 7 I I 22 12a 55125l 99 249. 68 187. 74 342. 77 1.67 1007. 17 770. 91 56 338. 43 342. 8426. 37 44. 72

Total 5. 54 87. 86 252. 19 1689. 34 1632.00

EXAMPLE 2 15 EXAMPLE 3 A process for fractionation of the abovementioned typical feedstock may be conducted according to the FIG. 5flow sheet to obtain three primary products as follows:

1) Ethylbenzene equivalent to about 95% of that in the feedstock, at 99%minimum purity, containing a maximum of 0.5 mol percent total xylene.

(2) o-Xylene equivalent to about 90% of that in the feed stock, at 95minimum purity, containing a maximum of 4.0 mol percent other 08H10material and 1.0 mol percent heavy ends.

(3) A xylene isomer mixture, predominantly p-xylene and m-xylene, atmaximum economic yield consistent with the stated production ofethylbenzene and o-xylene. For this case, the apparatus includes (l) aprefractionation column having about 74 theoretical stages, 100 actualtrays, a reux ratio of 5.8 (feed basis), operable at `61.1 p.s.i.a.(accumulator pressure), and capable of a heat input of 54.2 MMB.t.u./hr.; (2) a multi-section complex fractionati-ng column havingabout 298 theoretical stages, 410 actual trays divided into a lowersection comprising a column of 75 trays, a first intermediate sectionalso comprising a column of 75 trays, a second intermediate sectioncomprising a column of 130 trays, and an upper section also comprising acolumn of 130 trays, having a reflux ratio of 11.2 (total feed basis),operable at 14.7 p.s.i.a. (accumulator pressure), and capable of a heatinput of 57.2 MM B.t.u./hr. in a bottom reboiler plus 54.2 MM B.t.u./hr.transferred from the prefractionation column in an intermediatereboiler; and (3) an o-xylene purification column, having about `65theoretical stages, 100 actual trays, a reux ratio of 5.4, operable at14.7 p.s.i.a., and capable of a heat input of 15.3 MM B.t.u./hr. In thiscase, an ethylbenzene purification column is not required inasmuch asthe overhead product yfrom the complex fractionator meets theethylbenzene product specifications. Other units for p-xylene separationand xylenes isomerization also are not required to obtain the desiredproducts. A material balance at various points in the process is givenin Table 2.

A process for fractionation and conversion of the above mentionedtypical feedstock may be conducted according to the FIG. 3 flow sheet toobtain three primary products as follows:

(l) Ethylbenzene equivalent to about 95% of that in the feedstock, at99% minimum purity, containing a maximum of 0.5 mol percent light endsand 0.5 mol percent total xylene.

(2) o-Xylene at maximum economic yield and 95% minimum purity,containing a maximum of 4.0 mol percent other CgHm material and 1.0 molpercent heavy ends.

y(3) p-Xylene at maximum economic yield and 99% minimum purity,consistent Iwith the stated production of ethylbenzene and o-Xylene. Forthis case, the apparatus includes (l) a prefractionation column havingabout 73 theoretical stages, 105 actual trays, a reflux ratio of 5.7(feed basis), operable at 50.5 p.s.i.a. (accumulator pressure), andcapable of a heat input of 54.0 MM B.t.u./ hr.; (2) a multisectioncomplex fractionating column having about '244 theoretical stages, 350actual trays divided into a lower section comprising a column of trays,a irst intermediate section also comprising a column of 80 trays, asecond intermediate section comprising a column of 100 trays, and anupper section comprising a column of trays, having a reflux ratio of20.4 (total yfresh feed basis), operable at 14.7 p.s.i.a. (accumulatorpressure), and capable of a heat input of 152.3 MM B.t.u./hr. in abottom reboiler plus 54.0 MM B.t.u./hr. transferred from theprefractionation column in an intermediate reboiler; (3) an o-xylenepurification column having about 62 theoretical stages, 90 actual trays,a reflux ratio of 5.3, operable at 14.7 p.s.i.a., and capable of a heatinput of 27.1 MM B.t.u./hr.; (4) an ethylbenzene purification unit inwhich ethylbenzene is separated by extractive distillation; (5) ap-xylene separation unit in which p-Xylene is separated by selectiveadsorption; and (6) a xylene isomerization unit in which m-xylene isconverted to additional p-xylene and o-xylene, conversion-generatedlight ends are-separated and the net isomerate is recycled to thecomplex fractionating column by being combined (in this case) with theprefractionation column bottoms. A material balance at various points inthe process is given 'in Table 3.

TABLE 2 o-Xylene purification Prefractionator Complex fractionatorcolumn (column 49) (column 2) (unit 16) Feed Over- Over- Sideo-XyleneHea stock head Bottoms head stream Bottoms products enilss Component,pound mols per hour: (Line 43% (Line (Line 3) (Line l) (Line 19) (Line15) (Line 18) (Line 17) 91s4 Qooo "ifi' 87125 "ES'IIIIII 96. 09 64. 273l. S2 40 95. 39 30 241. 12 129. 7S 111. 34 03 237. 03 4. 06 117. 08 2.34 114. 74 9. 56 107. 52 32. 97 32. 97 32. 97

HN @HaHa/HN@ wm .H H .Rw HHH .wv n@ .mH @n .QS Ho .www S .HHN @o .com.mow @H om .www Ho .Ham .Hm .n I H205 -y 5.@ @Hlm HHH .-..lll HHHH`llillllllm 7.5.1---- 55m mH NMS owdH mm.mH mo. @ESN HHNMH ---1.1.1. H HHmd woHHH lllll.. .1..-.,ilEHHn .l mo. 1.1-1-1.-- oms @H @om wmmm moH om#mdmn mo. NHNH oHmmH mHHH 1li,.-.1212.-.-..,.I-@HSHHUE low. .--.IllllmH.. NQHHNH no.: Hmdo mn. mm ov. moHm cs w @edm{11:72.21}-.-11-11.22:@HSNAH mm .hm Ho N@ .Hm mw .mm Ho Nm .mm mH w .Hco .om .Hm :@HSNEHHHHHHMH om. mH. HH.H mo. QH.. I E.. mw. l hm. IIHD EenEn wHoE mHzEoHH EEQQEQO We claim:

1. The process for fractionating predominantly C8 aromatic hydrocarbonmixtures containing ethylbenzene and xylenes into component fractions,that includes feeding a first C8 aromatics stream relatively rich inethylbenzene into a multilevel fractionation zone at a first feed traylocated intermediate the top and bottom of said zone, feeding a secondC8 aromatics feed stream relatively rich in o-xylene into said zone at asecond feed tray located intermediate the top and bottom of said zoneand below said rst feed tray, and heating both feed streams with acommon heat ow in the multilevel fractionation zone, and separating apredominantly ethylbenzene overhead stream, a predominantly o-Xylenebottom stream and a xylene isomer sidestream at a level of said zonebetween said first and second feed trays.

2. Process according to claim 1 in which said fractionation zone isoperated at temperatures between 250 and 500 F., and pressures betweenl0 and 100 p.s.i.a.

3. Process according to claim 1 in which said first feed streamcomprises at least 8 mol percent ethylbenzene.

4. Process according to claim 1 in which said second feed streamcomprises at least 8 mol percent o-xylene.

5. The process for fractionating predominantly C8 aromatic hydrocarbonmixtures containing ethylbenzene and xylenes into component fractions,that includes feeding a first C8 aromatics stream relatively rich inethylbenzene into a multilevel fractionation zone at a first feed traylocated intermediate the top and bottom of said zone, feeding a secondC8 aromatics feed stream relatively rich in o-Xylene into said zone at asecond feed tray located intermediate the top and bottom of said zoneand below said first feed tray, heating both feed streams with a commonheat flow in the multilevel fractionation zone, and separating apredominantly ethylbenzene overhead stream, a predominantly o-xylenebottom stream and a xylene isomer sidestream at a level of said zonebetween said first and second feed trays, processing said xylene isomersidestream to isolate p-xylene therefrom, processing the resultingp-xylene depleted sidestream in a conversion zone to convert m-xyleneisomer present therein to other Xylene isomers, and returning conversionzone efliuent to the fractionation zone as said second feed stream.

6. Process according to claim 5 in which said fractionation zone isoperated at temperatures between 250 and 500 F., and pressures betweenl0 and 100 p.s.i.a.

7. Process according to claim S in which said first feed streamcomprises at least 8 mol percent ethylbenzene.

8. Process according to claim 5 in which said second feed streamcomprises at least 8 mol percent o-xylene.

9. Process according to claim 5 including converting m-xylene too-xylene in said conversion zone.

10. Process according to claim 5 including converting m-xylene top-xylene in said conversion zone.

11. Process according to claim 5 in which said common heat fiow in thefractionation zone is utilized to separate from each other o-xylene andp-xylene produced in the conversion zone.

12. Process according to claim 5 in which the amount of p-xyleneisolated from the sidestream is at least equal to the amount of p-xylenepresent in the first feed stream.

13. Process according to claim S including converting o-xylene top-xylene in said conversion zone.

14. Process according to claim 5 including converting p-Xylene too-xylene in said conversion zone.

15. Process according to claim 13 including returning separated o-xyleneto the conversion zone for conversion into p-xylene.

16. The process lfor fractionating a predominantly C8 aromatichydrocarbon mixture containing ethylbenzene and xylenes into componentfractions, that includes heating said mixture in a preliminaryfractionation zone, and separating said mixture into an overhead portionrelatively rich in ethylbenzene and a bottom portion relatively rich ino-xylene, feeding said overhead portion into a multilevel fractionationzone at a first feed tray located intermediate the top and bottom ofsaid zone, feeding said bottom portion into said multilevel zone at asecond feed tray located intermediate the top and bottom of said zoneand below said first feed tray, and heating both feed streams with acommon heat ow in the multilevel fractionation zone, and separating apredominantly ethylbenzene overhead stream, a predominantly o-xylenebottom stream and a xylene isomer sidestream at a level of saidmultilevel zone between said first and second feed trays.

17. Process according to claim 16 in which said multilevel fractionationzone is operated at temperatures between 250 and 500 F. and pressuresbetween 10 and 100 p.s.i.a.

18. Process according to claim 16 in which said first `feed stream tothe multilevel fractionation zone cornprises at least 8 mol percentethylbenzene.

19. Process according to claim 16 in which said second feed stream tothe multilevel fractionation zone comprises at least 8 mol percento-xylene.

20. Process according to claim 16 in which said preliminaryfractionation zone is operated at temperatures between 250 and 600 F.and pressures between l0 and 150 p.s.i.a.

21. Process according to claim 16 including transferring heat from saidpreliminary fractionation zone to said fractionation zone.

22. Process according to claim 21 in which said heat transfer iseffected by cycling said preliminary fractionation zone overhead throughthe fractionation zone above the sidestream draw from said zone.

23. Process according to claim 22 including also incrementally heatingthe first stream in the fractionation zone to separate ethylbenzene andlight ends from the pxylene and m-xylene in said first stream.

24. Process according to claim 16 including also reboiling hydrocarbonsin the multilevel fractionation zone between the feed streams againstcondensation of preliminary fractionation zone overhead vapors outsidesaid fractionation zone.

25. Process according to claim 24 including drawing hydrocarbons forreboiling from the fractionation zone at the level of said sidestream,whereby said preliminary fractionation zone heat is utilized in bothstripping and rectification operations in the multilevel fractionatingzone.

26. The process for fractionating a predominantly C8 aromatichydrocarbon mixture containing ethylbenzene and xylene into componentfractions, that includes heating said mixture in a preliminaryfractionation zone, and separating said mixture into an overhead portionrelatively rich in ethylbenzene and a bottom portion relatively rich ino-xylene, feeding said overhead portion into a multilevel fractionationzone at a first feed tray located intermediate the top and bottom ofsaid zone, feeding said bottom portion into said zone at a second feedtray located intermediate the top and bottom of said zone and below saidfirst feed tray, heating both feed streams with a common heat flow inthe multilevel fractionation zone, and separating a predominantlyethylbenzene overhead stream, a predominantly o-xylene bottom stream anda xylene isomer sidestream at a level of said zone between said firstand second feed trays, processing said xylene isomer sidestream toisolate p-xylene therefrom, processing the resulting p-xylene depletedsidestream in a conversion zone to convert m-xylene isomer presenttherein to other xylene isomers, and returning conversion zone effluentto the multilevel fractionation zone below said sidestream level.

27. Process according to claim 26 in which said multilevel fractionationzone is operated at temperatures between 250 and 500 F. and pressuresbetween l0 and p.s.i.a.

28. Process according to claim 26 in which said overhead portion feedstream to the multilevel fractionation zone comprises at least 8 molpercent ethylbenzene.

29. Process according to claim 26 in which said bottom portion feedstream to the multilevel fractionation zone comprises at least 8 molpercent o-xylene.

30. Process according to claim 26 in which said preliminaryfractionation zone is operated at temperatures between 250 and 600 andpressures between l0 and 150 p.s.i.a.

31. Process according to claim 26 including transferring heat from saidpreliminary fractionation zone to said fractionation zone.

32. Process according to claim 31 in which said heat transfer iseffected by cycling said preliminary fractionation zoneoverhead throughthe fractionation zone above the sidestream draw from said zone.

33. Process according to claim 26 including also reboiling hydrocarbonsin the multilevel fractionation zone between the feed streams againstcondensation of preliminary fractionation zone overhead vapors outsidesaid fractionation zone.

34. Process according to claim 33 including drawing hydrocarbons forreboiling from the fractionation zone at the level of said sidestream,whereby said preliminary fractionation zone heat is utilized in bothstripping and rectification operations in the multilevel fractionatingzone.

35. Process according to claim 26 including converting m-Xylene too-xylene in said conversion zone.

36. Process according to claim 26 including converting m-xylene top-xylene in said conversion zone.

37. Process according to claim 26 in which said common heat flow in thefractionation zone is utilized to separate from each other o-Xylene andp-xylene produced in the conversion zone.

38. Process according to claim 26 in which the amount of p-xyleneisolated from the sidestrea-m is at least equal to the amount ofp-xylene present in the first feed stream.

39. Process according to claim 26 including converting p-xylene too-xylene in said conversion zone.

40. Process according to claim 26 including converting o-xylene top-xylene in the conversion zone.

41. Process according to claim 40 including also returning separatedo-Xylene to the conversion zone for conversion into p-xylene.

42. The process for fractionating a predominantly C8 aromatichydrocarbon mixture containing ethylbenzene and xylenes into componentfractions, that includes depleting said mixture of m-xylene, feeding afirst feed stream comprising a m-xylene depleted C8 aromatic mixture andrelatively rich in ethylbenzene into a multilevel fractionation zone ata first feed tray located intermediate the top and bottom of said zone,feeding a second C8 aromatic feed stream comprising C8 aromatics andrelatively rich in o-xylene into said zone at a second feed tray locatedintermediate the top and bottom of said zone and below said first feedtray, and heating both feed streams with a common heat flow in themultilevel fractionation Zone, and separating a predominantlyethylbenzene overhead stream, a predominantly o-xylene bottom stream anda Xylene isomer sidestream at a level of said zone between said firstand second feed trays.

43. yProcess according to claim 42 in which said multilevelfractionation zone is operated at temperatures between 250 and 500 F.and pressures between 10 and 100 p.s.i.a.

44. Process according to claim 42 in which said first feed stream to themultilevel fractionation zone cornprises at least 8 mol percentethylbenzene.

45. Process according to claim 42 in which said second feed stream tothe multilevel fractionation zone cornprises at least 8 mol percento-xylene.

46. Process according to claim 42 including also chemg ically modifyingthe m-xylene to a form separable from 20 the first feed stream tofacilitate m-xylene depletion therefrom.

47. Process according to claim 46 in which -said m- Xylene is complexedwith a complexing agent.

48. Process according to claim 47 in which said m-xylene is complexedwith H13-BFS.

49. Process according to claim 42 including also processing saidsidestream to isolate p-xylene therefrom and returning the p-xylenedepleted stream to said multilevel fractionation zone at a point belowsaid sidestream level, said p-xylene depleted stream comprising all orpart of said second feed stream to said fractionation zone.

50. Process according to claim 42 including also processing saidsidestream to isolate p-xylene therefrom and returning at least aportion of said p-Xylene depleted stream to the feed ahead of them-Xylene depleting zone.

51. Process according to claim 42 including also processing saidsidestream to isolate p-xylene therefrom and returning a portion of saidp-Xylene depleted stream to said multilevel fractionation zone at apoint abovel said sidestream level.

52. Process according to claim 42 including also converting m-xyleneseparated from said mixture in said depleting step to o-xylene andfeeding the o-xylene obtained to said multilevel fractionation zone.

53. Process according to claim 42 including also converting m-xyleneseparated from said mixture in said depleting step to p-xylene andfeeding the p-Xylene obtained to said multilevel fractionation zone.

S4. Process according to claim 42 including also converting m-xyleneseparated from said mixture in said depleting step into o-Xylene andp-xylene and in which said common heat fiow in the multilevelfractionation zone is utilized to separate from each other o-Xylene andp-Xylene so produced.

55. Process according to claim 42 in which the amount of p-Xyleneisolated from the sidestream is at least equal to the amount of p-xylenepresent in said mixture.

56. Process according to claim 42 including also converting separatedo-Xylene to p-xylene and feeding the p-xylene so produced to themultilevel fractionation zone.

57. Process according to claim 49 including also converting isolatedp-xylene to o-xylene and feeding the o-xylene so produced to themultilevel fractionation zone.

58. The process for fractionating a predominantly C8 aromatichydrocarbon mixture containing ethylbenzene and xylene into componentfractions, that includes chemically separating m-Xylene from the mixtureto deplete the feed of m-xylene, heating the m-Xylene depleted mixturein a preliminary fractionation zone, and separating said mixture into anoverhead portion relatively rich in ethylbenzene and a bottom portionrelatively rich in o-Xylene, feeding said overhead portion into amultilevel fractionation zone as a first feed stream at a first feedtray located intermediate the top and bottom of said zone, feeding saidbottom portion into said zone as a second feed stream at a second feedtray located intermediate the top and bottom of said zone and below saidfirst feed tray, heating both feed streams with a common heat flow inthe multilevel fractionation zone, and separating a predominantlyethylbenzene overhead stream, a predominantly o-xylene bottom stream anda predominantly pxylene sidestream at a level of said zone between saidfirst and second feed trays.

59. Process according to claim 58 in which said multilevel fractionationzone is operated at temperatures between 250 and 500 F. and pressuresbetween l0 and l0() p.s.1.a.

60. Process according to claim 58 in which said first feed stream to themultilevel fractionation zone comprises at least 8 mol percentethylbenzene.

61. Process according to claim 58 in which said second feed stream tothe multilevel fractionation zone comprises at least 8 mol percento-xylene.

62. Process according to claim 58 in which said preliminaryfractionation zone is operated at temperatures tween 250 and `600" F.and pressures between and 1S() p.s.i.a.

63. Process according to claim 58 in which said m-xylene is complexedwith a complexin g agent.

64. Process according to claim 63 in which said m-xylene is complexedwith HF-BF3.

65. Process according to claim S8 including also processing saidsidestream to isolate p-xylene therefrom and returning at least aportion of the p-xylene depleted stream to said fractionation zone.

66. Process according to claim 58 including also processing saidsidestream to isolate p-xylene therefrom and returning at least aportion of the p-xylene depleted stream to said preliminaryfractionation zone.

67. Process according to claim 58 including also processing saidsidestream to isolate p-xylene therefrom and returning at least aportion of the p-xylene depleted stream to the feed ahead of them-Xylene depleting zone.

68. Process according to claim 58 including transferring heat from saidpreliminary fractionation zone to said fractionation zone.

69. Process according to claim 68 in which said heat transfer iseffected by cycling said preliminary fractionation zone overhead throughthe fractionation zone above the sidestream draw from said zone.

70. Process according to claim 58 including also reboiling thehydrocarbon in the fractionation zone between the feed streams againstcondensation of preliminary fractionation zone overhead vapors outsidesaid fractionation zone. Y

:71. Process according to claim 70 including drawing hydrocarbons forreboiling from the fractionation zone at the level of said sidestreaim,whereby said preliminary 22 fractionation zone heat is utilized in bothstripping and rectification operations in the fractionating zone.

72. Process according to claim 58 including also converting m-xyleneseparated from said mixture to o-xylene and feeding the o-xyleneobtained to said multilevel fractionation zone.

73. Process according to claim 58 including also converting m-xyleneseparated from said mixture to p-xylene and feeding the p-xyleneobtained to said multilevel fractionation zone.

74. Process according to claim S8 including also converting m-xyleneseparated from said mixture into o-xylene and p-xylene and in which saidcommon heat flow in the multilevel fractionation zone is utilized toseparate from each other o-xylene and p xylene so produced.

75. Process according to claim in which the amount of p-.xylene isolatedfrom the sidestream is at least equal to the amount of p-xylene presentin said mixture.

76. Process according to claim 58 including also converting separatedo-xylene to p-xylene and feeding the p-xylene so produced to themultilevel fractionation zone.

77. Process according to claim 65 including also converting isolatedp-xylene to o-xylene and feeding the o-xylene so produced to themultilevel fractionation zone.

References Cited UNITED STATES PATENTS CURTIS R. DAVIS, Primary ExaminerU.S. Cl. XJR. 260-674A

